Avoid Bottlenose Dolphins (Tursiops Truncatus)
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Notes MARINE MAMMAL SCIENCE, 31(1): 386–397 (January 2015) Published 2014. This article is a U.S. Government work and is in the public domain in the USA. DOI: 10.1111/mms.12154 Acoustic evidence that harbor porpoises (Phocoena phocoena)avoid bottlenose dolphins (Tursiops truncatus) EIREN K. JACOBSON,1 Scripps Institution of Oceanography, University of California, San Diego, 9500 Gilman Drive, La Jolla, California 92093-0208, U.S.A.; KARIN A. FORNEY, Marine Mammal and Turtle Divison, Southwest Fisheries Science Center, National Marine Fisheries Service, National Oceanic and Atmospheric Administration, 110 Shaffer Road, Santa Cruz, California 95060, U.S.A.; JAMES T. HARVEY, Moss Landing Marine Laboratories, 8272 Moss Landing Road, Moss Landing, California 95039, U.S.A. Along the West Coast of the United States, harbor porpoises (Phocoena phocoena) and coastal common bottlenose dolphins (Tursiops truncatus) overlap in distribution from Point Conception to San Francisco Bay in a narrow coastal band (Wells et al. 1990; Hansen and Defran 1993; Carretta et al. 1998, 2001). This overlap is relatively recent. Increased water temperatures in California during the 1982–1983 El Nino~ event prompted a northward range expansion of coastal bottlenose dolphins past Point Conception into central California (Hansen 1990, Wells et al. 1990). Harbor porpoises occur in distinct populations along the California coast (Calambokidis and Barlow 1991, Chivers et al. 2002, Carretta et al. 2009), whereas coastal bottlenose dolphins move throughout their range and comprise a single population (Defran et al. 1999, Hwang et al. 2014). In California and in other regions where these species overlap, interspecies aggres- sion has been observed (Ross and Wilson 1996, Dunn et al. 2002, Cotter et al. 2012). Between 1991 and 1993 in Moray Firth, Scotland, blunt force trauma associ- ated with bottlenose dolphin attacks was the largest contributor to stranded harbor porpoise mortality (Ross and Wilson 1996). Bottlenose dolphin attacks on harbor porpoises have also been documented in Cardigan Bay, Wales (Jepson and Baker 1998) and in Monterey Bay, California (Cotter et al. 2012, Wilkin et al. 2012). The first documented stranding of a harbor porpoise killed by bottlenose dolphins in Cali- fornia occurred in July 2005. Between 2007 and 2009 researchers witnessed three bottlenose dolphin attacks on harbor porpoises in Monterey Bay (Cotter et al. 2012). These attacks involved 23 individual bottlenose dolphins, 21 of which were known 1Corresponding author (e-mail: [email protected]). 386 NOTES 387 to be males. Observed attacks occurred in the late summer and early fall, during the possible breeding season for bottlenose dolphins. In Monterey and Santa Cruz counties between 2005 and 2011, 179 harbor por- poise strandings were documented. Of these, 43 of 68 stranded porpoises in which cause of death could be determined presented signs of blunt force trauma, at least 28 of which were consistent with attacks by bottlenose dolphins.2 In California between 1998 and 2010, 54 of 216 stranded porpoises in which cause of death was determined presented signs of blunt force trauma consistent with attacks by bottlenose dolphins (Wilkin et al. 2012). In 2007 an apparent increase in harbor porpoise strandings in central California triggered the declaration of an Unusual Mortality Event (UME) and the stranding rate was more than twice the mean annual rate of the previous dec- ade during 2008 and 2009. During this time, stranding rates were highest from June to November and blunt force trauma was the most common diagnosed cause of death (Wilkin et al. 2012). Taken together, the findings from California suggest that in this region a relatively sudden onset of aggression towards harbor porpoise was exe- cuted by a few male bottlenose dolphins. Hypothesized drivers for bottlenose dolphin attacks on harbor porpoise can be broadly divided into ecological and behavioral explanations (Ross and Wilson 1996, Cotter et al. 2012). Possible ecological drivers include prey competition and feeding interference, whereas possible behavioral drivers include object-oriented play to prac- tice fighting or infanticidal behaviors, with high levels of testosterone and a skewed sex ratio perhaps contributing to heightened aggression in male bottlenose dolphins (Ross and Wilson 1996, Cotter et al. 2012). All of the proposed ecological explanations hinge on some form of competition for resources, for which evidence in California is weak. Spitz et al. (2006) reported quali- tative similarities in diet composition of bottlenose dolphins and harbor porpoise in the Bay of Biscay, but quantitative examination found strong evidence that diet pro- files were distinct between these two species. There has been no comprehensive com- parative study of the diets of bottlenose dolphins and harbor porpoise on the U.S. West Coast (Cotter et al. 2012). The most relevant available data indicate that bottle- nose dolphins in California feed primarily on surfperches (family Embiotocidae)and croakers (family Sciaenidae;Defranet al. 1999 and references therein), whereas harbor porpoises in this region feed mainly on market squid (Doryteuthis opalescens) and small forage fish (Cotter et al. 2012 and references therein). Harbor porpoise stomachs col- lected from stranded animals in Monterey Bay contained primarily northern anchovy (Engraulis mordax), spotted cusk-eel (Chilara taylori), rockfish (family Sebastes), and market squid (Dorfman 1990, Sekiguchi 1995, Byrd 2001). More recent analyses of stranded animals in central California confirmed northern anchovy, market squid, and sardine (Sardinops sagax) as harbor porpoise prey items.3 Although not conclusive, these data indicate that harbor porpoise are generalists and may have seasonal or inter- annual variation in preferred prey types. The limited evidence for dietary overlap in combination with known differences in depth distributions for these two species indi- cate that competition for resources is an improbable explanation for the observed bot- tlenose dolphin aggression in California. 2NOAA Fisheries West Coast Region, 501 West Ocean Boulevard, Suite 4200, Long Beach, CA 90802. Unpublished data obtained in July 2013. 3Michelle Berman, Department of Vertebrate Zoology, Santa Barbara Museum of Natural History, 2559 Puesta del Sol, Santa Barbara, CA 93105. Unpublished data obtained in February 2013. 388 MARINE MAMMAL SCIENCE, VOL. 31, NO. 1, 2015 The prevailing hypothesis for bottlenose dolphin aggression towards harbor porpoises is that it is either a misdirection of infanticidal tendencies or a way of devel- oping infanticidal behaviors (Patterson et al. 1998, Kaplan et al. 2009, Cotter et al. 2012). In Moray Firth, Scotland, stranded bottlenose dolphin calves presented inter- nal and external injuries consistent with infanticide (Patterson et al. 1998) at the same time that harbor porpoise strandings were dominated by dolphin-inflicted blunt force trauma (Ross and Wilson 1996). Infanticide also has been observed in bottle- nose dolphins in Virginia (Dunn et al. 2002) and Florida (Kaplan et al. 2009). It is likely that infanticidal behavior was previously observed in bottlenose dolphins but was documented by researchers as play rather than aggression (Dunn et al. 2002). In some of these instances, attacked individuals of both species have been similarly sized (100–150 cm; Ross and Wilson 1996, Patterson et al. 1998, Dunn et al.2002).This link with infanticide is supported by some observations that aggressors are males (Kaplan et al. 2009, Cotter et al. 2012) who may use infanticide as a strategy to gain reproductive access to females (Kaplan et al. 2009). In California 92% of bottlenose dolphin aggressors in observed attacks were known or putative males, and there was 40% overlap in bottlenose dolphin identity between attacks (Cotter et al. 2012). However, there may be other explanations for the observed interspecies aggression since not all stranded harbor porpoises with dolphin-inflicted injuries fall within this size range (Wilkin et al. 2012), bottlenose dolphins have been observed attacking other delphinids (e.g., Barnett et al. 2009), and infanticide has never been docu- mented in bottlenose dolphins in California (Cotter et al. 2012). In the present study, we investigated whether this documented conflict between bottlenose dolphins and harbor porpoises in California affected harbor porpoise behavior within the nearshore environment where habitat overlap occurs with bottle- nose dolphins. We expected to find lower densities of harbor porpoises when bottle- nose dolphins were present than when they were absent. We used data from moored click detectors (C-PODs; Chelonia Ltd., http:// www.chelonia.co.uk) which were deployed at a study site in Monterey Bay (Fig. 1) to quantify the relative presence of harbor porpoises as well as bottlenose dolphins. These data were collected as part of a separate study but provided an opportunity to examine the acoustic behavior of both species within a few kilometers of the sites where bottlenose dolphin attacks on harbor porpoises were observed by Cotter et al. (2012) in previous years. Our instruments were moored in Monterey Bay at 36°52053″N, 121°50031″W approximately 15 m deep and 6 m above the sea floor. We used a mooring design that required diver installation and retrieval but allowed C-PODs to be serviced by hand from a small boat. Our C-POD installation was active from 7 October to 30 December 2011, for a total of 2,016 one-hour